The presence of large deposits of oil shale in the semi-arid, high plateau region of the western United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising a marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded to form a fragmented permeable mass of particles containing oil shale. The fragmented mass is ignited near the top of the retort to establish a combustion zone. An oxygen-supplying gas such as air, air and steam, air diluted with off-gas, or air enriched with oxygen is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented permeable mass of particles in the retort. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. The retorting zone moves from the top to the bottom of the retort ahead of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423 to Donald E. Garrett.
As used herein, the term "retorting zone" refers to that portion of the retort where kerogen in oil shale is being decomposed to liquid and gaseous products, leaving residual carbonaceous material in the retorted oil shale. The term "combustion zone" refers to a portion of the retort where the greater part of the oxygen in the retort inlet mixture that reacts with the residual carbonaceous material in the retorted oil shale is consumed.
It has been found desirable in some embodiments to have an intact subterranean base of operation above the fragmented permeable mass of formation particles in an in situ oil shale retort. Such a base of operation facilitates the drilling of blastholes into underlying formation for forming the fragmented mass in the retort and facilitates ignition over the entire top portion of the fragmented mass. Additionally, having a base of operation above the fragmented mass permits control of introduction of oxygen-supplying gas into the retort, provides a location for testing properties of the fragmented mass, such as distribution of void fraction, and provides a location for evaluation and controlling performance of the retort during operation.
The base of operation is separated from the retort by a layer of unfragmented formation extending between the top boundary of the retort and the floor of such a base of operation. The layer of unfragmented formation is termed a "sill pillar" which acts as a barrier between the in situ oil shale retort and the base of operation during retorting operations. It is, therefore, important that the sill pillar remain structurally sound, both for supporting the base of operation and for preventing entry of heat and gases into the base of operation during the retorting process.
Techniques for forming an in situ oil shale retort containing a fragmented permeable mass of formation particles and having a sill pillar of unfragmented formation between the top of the fragmented mass and an overlying base of operation are described in U.S. Pat. No. 4,118,071 by Ned M. Hutchins and in U.S. Pat. No. 4,192,554 by Thomas E. Ricketts. U.S. Pat. Nos. 4,118,071 and 4,192,554 are incorporated herein by this reference. The in situ oil shale retort formed by the method disclosed in U.S. Pat. No. 4,192,554 may not be completely full of oil shale particles, i.e., there can be a void space or plenum between the upper surface of the fragmented mass of oil shale particles and the top boundary of the retort.
In retorts where no open base of operation is provided, the formation overlying the fragmented permeable mass of formation particles extends all the way to the ground surface. In such an embodiment, blastholes are drilled through the overlying formation from the ground surface.
Examples of other techniques used for forming in situ oil shale retorts are described in U.S. Pat. No. 4,043,595 by French; U.S. Pat. No. 4,043,596 by Ridley; U.S. Pat. No. 4,043,597 by French; and U.S. Pat. No. 4,043,598 by French et al, each of which is incorporated herein by this reference.
In the past, a variety of techniques have been developed for igniting oil shale particles in an in situ oil shale retort in order to establish a combustion zone. Such techniques are disclosed in U.S. Pat. No. 3,952,801 and U.S. Pat. No. 3,990,835, both by Robert S. Burton, III. According to the techniques disclosed in these patents, a hole is bored to the top of the fragmented permeable mass of oil shale particles and a burner is lowered through the borehole to the oil shale to be ignited. A mixture of combustible fuel, such as LPG (liquefied petroleum gas), diesel oil, or shale oil, and oxygen-containing gas, such as air, is burned in the burner to provide a hot ignition gas which is introduced into the fragmented mass of oil shale particles. The burning is continued until a substantial portion of the oil shale has been heated above its self-ignition temperature so that combustion of the oil shale in the fragmented mass is self-sustaining after ignition. Thereafter, the burner is extinguished and an oxygen-supplying gas is introduced into the retort to advance the combustion zone through the fragmented mass.
As mentioned above, retorts have been formed with a plenum or void space between the top surface of the fragmented mass of oil shale particles in the retort and overlying unfragmented formation.
To ignite such a retort, hot ignition gases, which can comprise oxygen if desired, can be introduced into the plenum through one or more generally vertical boreholes through overlying unfragmented formation. Alternatively, if desired, a lateral drift which communicates with the top region or surface of the fragmented mass via the plenum can be formed through a side boundary of the retort. Hot ignition gases can be introduced into the plenum through the lateral drift.
Ignition gases introduced into the plenum for heating the fragmented mass surface can also heat the bottom surface of unfragmented formation overlying the plenum. Such heating of overlying formation can result in spalling or sloughing of the formation onto the surface of the fragmented mass in the retort. For instance, hot gases used for ignition can be at a temperature of 1000.degree. F. or more and it has been found that when unfragmented oil shale formation is heated to more than about 300.degree. F., spalling and sloughing is enhanced.
When the material which has sloughed into the retort plenum is heated sufficiently in the presence of oxygen, it burns. Burning of such sloughed material causes increased heating of overlying unfragmented formation and results in furthering the sloughing of such formation into the retort.
When formation which has sloughed into the retort plenum burns, it consumes an indeterminable amount of oxygen in the inlet gas introduced into the retort for ignition and/or retorting. This upsets the material balance in the retort and results in operational problems.
Additionally, when a retort has a sill pillar, and a void space exists between the bottom of the sill pillar and the top surface of the fragmented mass, the sill pillar can fail if sufficient formation sloughs from its bottom. Failure of such a sill pillar can allow heat and gases to escape from the retort into the base of operation, rendering the base of operation uninhabitable and, thus, useless for control and operation of the retort.
It is, therefore, desired to provide a process for enhancing the efficient ignition of a fragmented mass in a retort, while at the same time inhibiting overlying formation from sloughing into the retort plenum.